Metal-assisted Wet Chemical Etching Research Articles

The nature of photocatalytic hydrogen generation on silicon nanowires prepared by MAWC

Highly n-doped silicon nanowires (SiNWs) exhibit excellent hydrogen generation. Our results indicate three hydrogen generation possibilities, i.e. SiNWs oxidation, photocatalysis and the cleavage of dangling H bonds, are jointly responsible for the efficient hydrogen generation. Oxidation accounts for about 80% of total hydrogen, photocatalysis contributes less than 20% of total hydrogen, and about 0.1% of total hydrogen is from cleavage of dangling H bonds. SiNWs were oxidized in water and form SiOx (0 < X < 2) thin layer. Furthermore, the ratio of photo-generated hydrogen to photo-generated oxygen is about 2.4:1. It suggests that photocatalysis appears to be a process of water splitting. This study is significant to reveal the mechanism of hydrogen production on SiNWs prepared by Metal-Assisted Wet Chemical Etching (MAWC).

Deep-reactive ion etching of silicon nanowire arrays at cryogenic temperatures

The pursuit of sculpting materials at increasingly smaller and deeper scales remains a persistent subject in the field of micro- and nanofabrication. Anisotropic deep-reactive ion etching of silicon at cryogenic temperatures (cryo-DRIE) was investigated for fabricating arrays of vertically aligned Si nanowires (NWs) of a large range of dimensions from micrometers down to 30 nm in diameter, combined with commonly used wafer-scale lithography techniques based on optical, electron-beam, nanoimprint, and nanosphere/colloidal masking. Large selectivity of ∼100 to 120 and almost 700 was found with resists and chromium hard masks, respectively. This remarkable selectivity enables the successful transfer of patterned geometries while preserving spatial resolution to a significant extent. Depending on the requirements by applications, various shapes, profiles, and aspect ratios were achieved by varying process parameters synchronously or asynchronously. High aspect ratios of up to 100 comparable to the best result by metal-assisted wet-chemical etching and sub-μm trenches by DRIE were obtained with NW diameter of 200 nm, at an etch rate of ∼4 μm/min without being collapsed. At the same time, low surface roughness values were maintained on the NW top, sidewall, and bottom surface of ∼0.3, ∼13, and ∼2 nm, respectively, as well as high pattern fidelity and integrity, which were measured using angle-resolved Fourier microscopy, combined atomic force, and scanning electron microscopy on selected NWs. This work establishes the foundation in the controllable development of Si nanoarchitectures, especially at sub-100 nm structures, for energy-harvesting and storage, damage-free optoelectronics, quantum, photovoltaics, and biomedical devices.

Properties of phosphoric acid doped crystalline silicon with different light-trapping schemes for solar cells

Properties of phosphoric acid (H3PO4)-doped crystalline silicon (c-Si) with different light-trapping schemes have been investigated for solar cells. Microtextures, b-Si nanotextures, and hybrid textures are fabricated using sodium hydroxide (NaOH) and metal-assisted wet chemical etching (MACE). MACE has been achieved via 30 s of silver nanoparticles (Ag NPs) deposition in a solution of 4.8 M HF:18 mM AgNO3 and 20 s etching in a solution of HF:H2O2:DI H2O (2:1:5). Spin coating of H3PO4 (15% by volume) and Butanol (C4H10O; 75% by volume) on planar c-Si, microtextures, b-Si nanotextures, and hybrid textures is adopted to realize the front junction for the solar cells. Hybrid textures with nanotextures' heights of ∼450–550 nm and widths of ∼80–100 nm on microtextures present a weighted average reflection (WAR%) of 8.3% and an estimated junction depth of about ∼450 nm after the diffusion process. Sheet resistance (Rs) of 148 ± 1.3 Ω/□, carrier concentration (ne) of −6.01 × 1018 cm−3, and carrier mobility (μe) of 140.7 cm2/Vs is obtained for the hybrid textures. Solar cell with hybrid texturing achieves a maximum short-circuit current density (Jsc(max)) of 10.7 mA/cm2 owing to enhanced light-trapping, which results from optimum absorption. Open-circuit voltage (Voc), fill factor (FF), and efficiency (EFF) of 640 mV, 0.36, and 2.5% have been obtained for the hybrid textures solar cell compared to 600 mV, 0.34, 1.7% achieved in the b-Si nanotextures solar cell, respectively.

Dps protein localization studies in nanostructured silicon matrix by scanning electron microscopy

The present work is related to the microscopic studies of the morphology of the planar and inner part of silicon nanowires arrays before and after immobilization with a natural nanomaterial, Dps protein of bacterial origin. Silicon nanowires were formed by metal-assisted wet chemical etching. To obtain the recombinant protein, Escherichia coli cells were used as excretion strain and purification were carried out using chromatography. The combination of silicon nanowires with protein molecules was carried out by layering at laboratory conditions followed by drying under air. The resulting hybrid material was studied by high-resolution scanning electron microscopy. Studies of the developed surface of the nanowires array were carried out before and after combining with the bioculture. The initial arrays of silicon wireshave a sharp boundaries in the planar part and in the depth of the array, transition layers are not observed. The diameter of the silicon nanowires is about 100 nm, the height is over a micrometer, while the distances between the nanowires are several hundred of nanometers. The pores formed in this way are available for filling with protein during the immobilization of protein.The effectiveness of using the scanning electron microscopy to study the surface morphology of the hybrid material “silicon wires – bacterial protein Dps” has been demonstrated. It is shown that the pores with an extremely developed surface can be combined with a bio-material by deposition deep into cavities. The protein molecules can easily penetrate through whole porous wires matrix array. The obtained results demonstrate the possibility of efficient immobilization of nanoscaled Dps protein molecules into an accessible and controllably developed surface of silicon nanowires.

Ordered arrays of Si nanopillars with alternating diameters fabricated by nanosphere lithography and metal-assisted chemical etching

Ordered Si nanopillar arrays have a great potential for e.g. photonic, sensing and electronic devices. In the present article, we investigate the tailoring of Si nanostructure geometries obtained on large areas by nanosphere lithography combined with metal-assisted wet-chemical etching. In particular, the formation of Si nanopencils and, as a new feature of metal-assisted chemical etching, of nanopillars with single and multiple constrictions is demonstrated, which is accomplished by varying the HF/H2O2 ratio of the etching solution without modifying the noble metal film. As a result, it is possible to obtain defined constrictions in high aspect ratio Si nanopillars (length-to-diameter up to 27) with relative diameter reductions of up to around 0.6. Moreover, by controlled oxidation in water vapor nanoscale Si inclusions surrounded by an amorphous SiO2 shell are obtained. The morphology and structure of these pillar arrays are analyzed by scanning and transmission electron microscopy, complemented by optical reflection measurements. For the pencil-shaped Si nanostructures a reflectivity lower than 3% is found in the visible and infrared wavelength ranges. Nanopillars with multiple constrictions exhibit a significantly reduced thermal conductivity due to phonon backscattering. Therefore, the presented method could be implemented in the large-scale fabrication of advanced thermoelectric devices.

Peculiarities of electronic structure and composition in ultrasound milled silicon nanowires

The combined X-ray absorption and emission spectroscopy approach was applied for the detailed electronic structure and composition studies of silicon nanoparticles produced by the ultrasound milling of heavily and lowly doped Si nanowires formed by metal-assisted wet chemical etching. The ultrasoft X-ray emission spectroscopy and synchrotron based X-ray absorption near edges structure spectroscopy techniques were utilize to study the valence and conduction bands electronic structure together with developed surface phase composition qualitative analysis. Our achieved results based on the implemented surface sensitive techniques strongly suggest that nanoparticles under studies show a significant presence of the silicon suboxides depending on the pre-nature of initial Si wafers. The controlled variation of the Si nanoparticles surface composition and electronic structure, including band gap engineering, can open a new prospective for a wide range Si-based nanostructures application including the integration of such structures with organic or biological systems.

Silver mirror reaction as a simple method for silicon nanowires functionalization

A cheap and facile wet-chemical ‘silver mirror’ reaction approach has been proposed for the silicon nanowires (SiNWs), formed by the metal-assisted wet-chemical etching, decoration with silver nanoparticles (AgNPs). Scanning electron microscopy studies show that AgNPs can be deposited uniformly on the surface of SiNWs by the variation of AgNO3 concentration and reaction time. Adopting the reaction conditions, a homogenous AgNPs shell is formed around the silicon nanowires. The resultant AgNPs@SiNWs nanostructures show significantly (up to 40%) higher light absorption in the near-infrared range (up to 1500 nm) in comparison to SiNWs without AgNPs. This approach opens a new perspectives for a cheap and fast preparation of AgNP shells around SiNWs. Our observation supports that synthesized nanostructures might have promising application potential in near-infrared photodetectors or as photocatalysts.

Silicon Powder-Based Wafers for Low-Cost Photovoltaics: Laser Treatments and Nanowire Etching

In this study, laser-treated polycrystalline Si (pc-Si) wafers, fabricated by wire sawing of hot-pressed ingots sintered from Si powder, have been investigated. As-cut wafers and those with high-quality thin Si layers deposited on top of them by e-beam have been subjected to laser irradiation to clarify typical trends of structural modifications caused by laser treatments. Moreover, possibility to use laser-treated Si powder-based substrates for fabrication of advanced Si structures has been analysed. It is established that (i) Si powder-based wafers with thicknesses ~180 μm can be fully (from the front to back side) or partly (subsurface region) remelted by a diode laser and grain sizes in laser-treated regions can be increased; (ii) a high-quality top layer can be fabricated by crystallization of an additional a-Si layer deposited by e-beam evaporation on top of the pc-Si; and (iii) silicon nanowires can be formed by metal-assisted wet chemical etching (MAWCE) of polished Si powder-based wafers and as-cut wafers irradiated with medium laser power, while a surface texturing on the as-cut pc-Si wafers occur, and no nanowires can form in the region subject to a liquid phase crystallization (LPC) caused by high-power laser treatments.

Tailored Interfaces of the Bulk Silicon Nanowire/TiO2 Heterojunction Promoting Enhanced Photovoltaic Performances.

We report significantly improved silicon nanowire/TiO2 n+–n heterojunction solar cells prepared by sol–gel synthesis of TiO2 thin film atop vertically aligned silicon nanowire arrays obtained by facile metal-assisted wet electroless chemical etching of a bulk highly doped n-type silicon wafer. As we show here, chemical treatment of the nanowire arrays prior to depositing the sol–gel precursor has dramatic consequences on the device performance. While hydrofluoric treatment to remove the native oxide already improves significantly the device performances, hydrobromic (HBr) treatment consistently yields by far the best device performances with power conversion efficiencies ranging between 4.2 and 6.2% with fill factors up to 60% under AM 1.5G illumination. In addition to yield high-quality and easy to produce solar cell devices, these findings regarding the surface treatment of silicon nanowires with HBr suggest that HBr could contribute to the enhancement of the device performance not only for solar cells but also for other optoelectronics devices based on semiconductor nanostructures.

Recycling of silicon: from industrial waste to biocompatible nanoparticles for nanomedicine

The formation of photoluminescent porous silicon (PSi) nanoparticles (NPs) is usually based on an expensive semiconductor grade wafers technology. Here, we report a low-cost method of PSi NPs synthesis from the industrial silicon waste remained after the wafer production. The proposed method is based on metal-assisted wet-chemical etching (MACE) of the silicon surface of cm-sized metallurgical grade silicon stones which leads to a nanostructuring of the surface due to an anisotropic etching, with subsequent ultrasound fracturing in water. The obtained PSi NPs exhibit bright red room temperature photoluminescence (PL) and demonstrate similar microstructure and physical characteristics in comparison with the nanoparticles synthesized from semiconductor grade Si wafers. PSi NPs prepared from metallurgical grade silicon stones, similar to silicon NPs synthesized from high purity silicon wafer, show low toxicity to biological objects that open the possibility of using such type of NPs in nanomedicine.

Metal-Free Black Silicon for Solar-powered Hydrogen Generation

Nanostructured black Si materials prepared by metal-assisted wet chemical etching (MaCE) using HF to drive the nanostructuring, have emerged as excellent light-harvesting materials for solar water splitting. Recently, their remarkable solar-to-hydrogen conversion capability was attributed to their surface oxidation mechanism. However, the same mechanism inhibits the interfacial kinetics within hours, and therefore they are deemed not suitable for a fully-functional hydrogen generating cell. Herein, we investigate the performance and interfacial kinetics of solar-to-hydrogen fuel conversion on black Si photocathodes that, for the first time, are free of any metal catalysts. We developed a near 100% metal-free Si interface by a HF-free etching approach. Systematic analysis and optimization of the nanostructure morphology and photoelectrochemical procedures supported continuous photoelectrochemical hydrogen generation for ∼250h, involving −1.18×104C.cm−2 of charge transfer at overpotentials as low as −0.2V vs. RHE. We find that this high-performance of bSi makes it attractive as a green solar-to-hydrogen platform.

Electronic levels in silicon MaWCE nanowires: evidence of a limited diffusion of Ag

Deep level transient spectroscopy (DLTS) was performed on lowly n-doped silicon nanowires grown by metal-assisted wet chemical etching (MaWCE) with silver as the catalyst in order to investigate the energetic scheme inside the bandgap. To observe the possible diffusion of atoms into the bulk, DLTS investigation was also performed on the samples after removing the nanowires. Two of the four energy levels observed in the nanowires were also detected inside the substrate. Based on these results and on literature data about deep levels in bulk silicon, some hypotheses are advanced regarding the identification of the defects responsible for the energy levels revealed.

Influence of surface pre-treatment on the electronic levels in silicon MaWCE nanowires

Deep level transient spectroscopy (DLTS) was performed on n-doped silicon nanowires grown by metal-assisted wet chemical etching (MaWCE) with gold as the catalyst in order to investigate the energetic scheme inside the bandgap. To observe the possible dependence of the level scheme on the processing temperature, DLTS measurements were performed on the nanowires grown on a non-treated Au/Si surface and on a thermally pre-treated Au/Si surface. A noticeable modification of the configuration of the energy levels was observed, induced by the annealing process. Based on our results on these MaWCE nanowires and on literature data about deep levels in bulk silicon, some hypotheses were advanced regarding the identification of the defects responsible of the energy levels revealed.

Atomic and electronic structure peculiarities of silicon wires formed on substrates with varied resistivity according to ultrasoft X-ray emission spectroscopy

Silicon wires arrays have been produced by metal-assisted wet chemical etching with the use of crystalline silicon substrates. The arrays and individual nanowires have been studied by scanning and transmission electron microscopy. The electronic structure and phase composition of the surface and near-surface layers of the arrays have been studied by ultrasoft X-ray emission spectroscopy. It is shown that the morphologically more developed sample formed on a substrate with low resistivity is considerably more strongly subject to oxidation with noticeable formation of phases of intermediate silicon oxides. The array of nanowires formed on a substrate with high resistivity also undergoes natural oxidation, but does so to a substantially lesser extent and, with increasing depth of analysis, mostly contains the phase of crystalline silicon constituting the bulk of the nanowires being formed.

Controlled Pore Formation on Mesoporous Single Crystalline Silicon Nanowires: Threshold and Mechanisms

Silicon nanowires are prepared by the method of the two-step metal-assisted wet chemical etching. We analyzed the structure of solid, rough, and porous nanowire surfaces of boron-doped silicon substrates with resistivities ofρ&gt; 1000 Ωcm,ρ= 14–23 Ωcm, andρ&lt; 0.01 Ωcm by scanning electron microscopy and nitrogen gas adsorption. Silicon nanowires prepared from highly doped silicon reveal mesopores on their surface. However, we found a limit for pore formation. Pores were only formed by etching below a critical H2O2concentration (cH2O2&lt;0.3 M). Furthermore, we determined the pore size distribution dependent on the etching parameters and characterized the morphology of the pores on the nanowire surface. The pores are in the regime of small mesopores with a mean diameter of 9–13 nm. Crystal and surface structure of individual mesoporous nanowires were investigated by transmission electron microscopy. The vibrational properties of nanowire ensembles were investigated by Raman spectroscopy. Heavily boron-doped silicon nanowires are highly porous and the remaining single crystalline silicon nanoscale mesh leads to a redshift and a strong asymmetric line broadening for Raman scattering by optical phonons at 520 cm−1. This redshift,λSi bulk=520 cm−1 →λSi nanowire=512 cm−1, hints to a phonon confinement in mesoporous single crystalline silicon nanowires.

Fabrication of an ant-nest nanostructure in polycrystalline silicon thin films for solar cells

A silicon nanostructure is fabricated in polycrystalline silicon thin films by metal-assisted wet chemical etching using thermally annealed silver nanoparticles as a catalyst. The Si nanostructure has an ant-nest feature. Solar cells based on the silicon nanostructure gives 90% short-circuit current enhancement compared to the cells based on the planar film. We also report that Al2O3 passivation of the silicon nanostructure provides a new approach to improving the polycrystalline silicon thin film’s electrical property. The open-circuit voltage of the Al2O3 passivated cells is improved by 70 mV.

Characterization of Si nanowires synthesized using metal-assisted wet-chemical etching

A synthesis of vertical silicon nanowire array through metal-assisted chemical etching of highly doped p-type silicon wafers (100) in a solution of hydrofluoric acid and silver nitrate has been proposed. . The influences of the growth parameters such as solution concentration, etching time have been investigated. In addition, we consider other common parameters like wafer resistivity and temperature, which rely on the silicon nanowires formation. The results indicate that the silicon nanowires retain the single crystalline structure and crystallographic orientation of the starting silicon wafer. Furthermore, They provide excellent antireflection property with a low reflection loss of 3% for incident light within the wavelength range of 200–900 nm. Such nanowire arrays may have potential applications as antireflection surface for silicon solar cells

Nanotechnology enhanced solar cells prepared on laser-crystallized polycrystalline thin films (

It is shown in this paper that indeed silicon nanotechnology can significantly improve the performance of solar cells made on low-cost materials like polycrystalline silicon (pc-Si) thin films formed on glass substrates. By proper engineering of the nanowires (cleaning of the metal contamination from the metal-assisted wet chemical etching and passivation by a-Si:H emitter as well as by an ultrathin Al2O3 tunnel layer on the emitter surface) and by proper design of the cell structure (superstrate configuration), the detrimental effects related to the nanowires can be avoided while the light trapping properties remain. In superstrate configuration with an Ag reflector, a prototype solar cell with efficiency of 10% has been realized on a pc-Si thin film with a thickness of only 8µm formed on glass, resulting from an enhanced photocurrent due to light trapping by silicon nanowires. Further optimization steps are discussed and targeted to improve the efficiency and the performance of the solar cells.

Properties of Si nanowires as a function of their growth conditions

Silicon nanowires physical properties strongly depend on their growth conditions, as already assessed. We report on the electrical properties of nanowires (NWs) grown by the vapor–liquid–solid (VLS) mechanism, one of the most established for NW growth, and by the more recent metal-assisted wet chemical etching (MaCE). Wet etching growth process promises to be an industrial advantageous way for growing Si NWs, because of its cheapness, fastness, relative easiness. The electronic level scheme in VLS grown, boron (B)- and phosphorus (P)-doped NWs has been experimentally investigated. We have demonstrated that the doping impurities induce the same shallow levels as in bulk silicon. The presence of two donor levels in the lower half-bandgap is also revealed, which has been successfully related to VLS growth details.We report, also, on the first results on the physical properties of Si NW arrays grown by MaCE, and compare them to those of VLS grown Si NWs.

A novel silicon nanostructure with effective light trapping for polycrystalline silicon thin film solar cells by means of metal-assisted wet chemical etching

Effective light trapping is required for poly-Si thin film solar cells to compensate for the moderate light absorption. Recent developments of light trapping in the poly-Si cell technology focus on random light scattering in the absorber layer by three means: glass substrate texture, silicon film etch-back texture, and plasmonic nanoparticles. Although silicon nanostructures like porous silicon, silicon nanowires, and silicon nanoholes demonstrate good optical properties, they are seldom considered for poly-Si thin film solar cells due to generally poor electronic properties and difficulty of fabrication. In this paper, the first poly-Si thin film solar cells with a novel silicon nanostructure are fabricated. The Si nanostructure is fabricated by means of metal-assisted wet chemical etching. Silver nanoparticles created by thermal annealing of evaporated silver thin film are used as etching catalyst. Light absorption for both glass-side and air-side illumination are significantly improved. The short-circuit current is enhanced by 21.0% as measured from the glass side and 53.5% from the air side. The open-circuit voltage is improved by 22 mV. It is first demonstration of working poly-Si thin film cells made of such material.